6-endo Versus 5-exo radical cyclization: streamlined syntheses of carbahexopyranoses and derivatives by 6-endo-trig radical cyclization

Three factors that can direct 6-endo radical cyclization over 5-exo ring closure: substitution at C-5, vinyl radical cyclization and ring strain, have been considered in the context of the preparation of carbapyranoses from carbohydrate derivatives. As a result, alkyl radicals in substrates containing a strain inducing 2,3-O-isopropylidene ring, and vinyl radical in non-strained compounds undergo a completely regioselective 6-endo-trig ring closure leading to carbasugar derivatives.     

Identification of radical species derived from allergenic 15-hydroperoxyabietic acid and insights into the behaviour of cyclic tertiary allylic hydroperoxides in Fe(II)/Fe(III) systems

To understand the skin sensitization mechanism of 15-hydroperoxyabietic acid, the major allergen in colophony, we first examined the formation of potential reactive radicals derived from its reaction with light, heat and TPP-Fe³⁺. Trapping with 1,1,3,3-tetramethylisoindolin-2-yloxyl nitroxide confirmed the formation of carbon-centred radicals derived from allyloxyl/allylperoxyl radicals as a consequence of the hydroperoxide scission. Particular interest was further given to the reactivity with Fe(II)/Fe(III) due to the biological importance of haem containing enzymes. Using a monocyclic 15-hydroperoxyabietic acid-like compound as a model of allergenic allylic hydroperoxides, we evidenced, by the ESR spin-trapping technique, the competition between carbon and oxygen-centred radicals formed in the presence of Fe(II)/Fe(III) in organic/aqueous media. We complemented the study by showing the possibility of formation, via a radical mechanism induced by ferric chloride, of an adduct between the allylic hydroperoxide and N-acetyl-cysteine ethyl ester. The results gave new knowledge on the possible generation of highly reactive radicals that could lead to the formation of antigenic structures.     

Regioselective synthesis of quinolone-annulated sulfur heterocycles by aryl radical cyclization

The tin hydride-mediated cyclizations of a number of sulfides and sulfones under mild, neutral conditions, have been investigated accompanied by some amount of β-scission product for sulfides. The sulfides were derived from 4-mercaptoquinolone and 2-bromobenzyl bromides by phase transfer catalyzed reaction and the corresponding sulfones were prepared by treatment of the sulfides with m-CPBA at room temperature. The sulfides and the corresponding sulfones were then reacted with ⁿBu₃SnH-AIBN to give regioselective quinolone-annulated sulfur heterocycles.     

Regioselective radical ring-opening reaction of bicyclo[4.2.0]octan-2-ones promoted by samarium(II) iodide

Radical ring-opening reactions of bicyclo[4.2.0]octanones, its C6 alkyl derivatives, and tricyclic ketones promoted by SmI₂ gave cyclohexanones via fission of the external cyclobutane bond. The CO₂Me, CN, and phenyl derivatives led to the production of the eight-membered ring compounds through cleavage of the central cyclobutane bond. Using this regioselective reaction, the synthesis of (±)-acorenone was achieved.     

Thiophenol-mediated intramolecular radical cyclization: an efficient method for the synthesis of benzoxocine derivatives

A new, efficient, high yielding method for the synthesis of benzoxocine derivatives has been developed via a thiophenol-mediated intramolecular 8-endo radical cyclization. This method allowed the synthesis of the backbone of several sesquiterpenes.     

Titanocene(III) mediated 8-endo radical cyclizations for the synthesis of eight-membered cyclic ethers

Titanocene(III) chloride (Cp₂TiCl) mediated 8-endo radical cyclizations towards the synthesis of eight-membered cyclic ethers have been described. Titanocene(III) chloride was prepared in situ from commercially available titanocene dichloride (Cp₂TiCl₂) and activated zinc dust in THF.     

Samarium(II)-induced coupling of acid chlorides with allylic halides

Samarium(II)-induced coupling of acid chlorides with allylic halides gave diallylated tertiary alcohols. Monoallylated allylic ketones could not be obtained.     

Stereoselective synthesis of (−)-tetrahydrolipstatin via a radical cyclization based strategy

An efficient and flexible approach for the total synthesis of (−)-tetrahydrolipstatin is described. The main features of the synthetic strategy are a stereocontrolled radical cyclization and the successful utilization of commercially available S-malic acid.     

A new synthesis of 2-aryl/alkylbenzofurans by visible light stimulated intermolecular Sonogashira coupling and cyclization reaction in water

A visible light induced rapid one pot intermolecular Sonogashira coupling and 5-endo-dig cyclization in water of ortho-halophenols and terminal alkynes catalyzed by [Pd] have been developed to furnish 2-aryl/alkyl benzofurans in good yields sans Ru or Ir complexes or any other additives.     

Regioselective synthesis of substituted 1-indanols, 2,3-dihydrobenzofurans and 2,3-dihydroindoles by electrochemical radical cyclization using an arene mediator

Electrochemical reduction of haloarenes carrying 2-(1-hydroxybut-3-enyl), 2-allyloxy or N-allyl-N-methylamino group in the presence of phenanthrene as a mediator generated the corresponding aryl radicals and gave the corresponding 5-exo cyclization products in good yields. Higher regio- and stereoselectivities than those of usual radical cyclization using AIBN-Bu₃SnH were achieved.     

Reactions of carbonate radical anion with amino-carboxylate complexes of manganese(II) and iron(III)

Abstract

The reaction kinetics of eight amino-carboxylate complexes of Fe(III) and Mn(II) with carbonate radical anion were studied using the pulse radiolysis method and UV-vis spectroscopy. Difference spectra revealed the formation of Fe(IV) and Mn(III) after reaction with CO₃^??. Spectral measurements revealed the first step to be the coordination of carbonate to the metal center. All of these led to the conclusion that the role of coordinated carbonate is essential to the electron transfer process by carbonate radical anion.     

An efficient method for construction of tetrahydroisoquinoline skeleton via double cyclization process using ortho-vinylbenzaldehydes and amino alcohols: application to the synthesis of (S)-cryptostyline II

Thermal double cyclization reaction using ortho-vinylbenzaldehyde and 3-aminopropanols proceeded smoothly to give isoquinoline derivatives via 6π-azaelectrocyclization pathway. The method was applied to the efficient synthesis of (S)-cryptostyline II.     

Thin-layer chromatography of tris(1,10-phenanthroline)iron(II) and tris(2,2′-bipyridine)iron(II) on Sephadex G gels

Summary Gel chromatographic behaviour of tris(1, 10-phenanthroline)iron(II), tris(2,2′-bipyridine)iron(II) and tris(glycinato)cobalt(III) on Sephadex G-10 or G-25 was investigated by TLC with 0.001–1.0M NaCl as the eluent. The zone shapes and R_M values of tris(1,10-phenanthroline)iron(II) and tris(2,2′-bipyridine)iron(II) were appreciably dependent on the sample and eluent concentration, while the neutral complex, tris(glycinato) cobalt(III), exhibited the round zones with constant R_M values. The order of R_M values was found to be tris(glycinato)cobalt(III<tris(2,2∔pyridine)iron(II)<tris-(1,10-phenanthroline)iron(II) in all systems studied, although the reverse trend was expected when assuming the chromatographic behaviour of solute compounds to be controlled by the “sieving effect”. The comparison of the behaviour on Sephadex G gels with that on CM-cellulose revealed that the predominant mechanism involved is not the sieving effect, but ion-exchange and/or hydrophobic interaction.     

Five-membered ring chelate complexes of Ni(II), Pd(II) and Pt(II) derived of di-(2-pyridyl)-N-ethylimine

Cis-diaquobis{di-(2-pyridyl)-N-ethylimine}nickel(II) chloride (2) was obtained from the reaction of di-(2-pyridyl)-N-ethylimine (1) and [NiCl₂dppe] [dppe = cis-1,2-bis(diphenylphosphino)ethylene] in a 2:1 ratio in hot acetonitrile. Cis-dichloro{di-(2-pyridyl)-N-ethylimine}palladium(II) (3) and cis-dichloro{di-(2-pyridyl)-N-ethylimine}platinum(II) (4) complexes were obtained from the reaction of MCl₂ (M = Pd, Pt) and (1) in equimolar ratio in hot acetonitrile. Compounds 1–4 were characterized by IR spectroscopy, elemental analysis, and mass spectrometry; the complexes 3 and 4 were characterized in solution by NMR. In addition, solid state structures of compounds 1–4 were determined using single crystal X-ray diffraction analyses. X-ray diffraction data of the complexes 3 and 4 showed a distorted square planar local geometry at palladium and platinum atoms with the chlorine atoms in a cis-coordination; in 2 a local octahedral geometry at nickel atom was observed. Complexes 3 and 4 are arranged as dimers with a M?M distance of 3.4567(4) Å (M = Pd) and 3.4221(4) Å (M = Pt), respectively; 2 consists of units linked by intermolecular hydrogen bonding.     

The novel approach to indole alkaloids by using Pd(II)-catalyzed cyclization

The synthesis of indole skeleton by using Pd(II)-catalyzed cyclization of the urethane has been achieved. The urethanes with allylic alcohol were converted into vinyl indolines in good yield. The vinyl indoline was transformed into some intermediates of indole alkaloids.     

Zn(II), Cd(II) and Hg(II) complexes with 1-(p-methoxybenzyl)-2-(p-methoxyphenyl)benzimidazole: Syntheses, structures and luminescence

Five coordination compounds Zn(mbmpbi)₂Cl₂ (1), Zn(mbmpbi)₂Br₂ (2), Cd(mbmpbi)₂Cl₂ (3), Hg(mbmpbi)₂Cl₂ (4) and Hg(mbmpbi)₂Br₂ (5) were synthesized by the reaction of 1-(p-methoxybenzyl)-2-(p-methoxyphenyl)benzimidazole (mbmpbi) with the corresponding metal halides. The complexes have been characterized by elemental analysis, conductance measurements, FT-IR, ¹H NMR and photoluminescence spectral studies. The ligand mbmpbi exhibits the N-benzimidazole coordination. The structures of 3-5 have been determined by single crystal X-ray diffraction. These three complexes are isostructural, crystallizing in the monoclinic system, P2/n space group with a distorted tetrahedral geometry around the metal ion. Zn(II) and Cd(II) complexes show strong blue emission in solid state at room temperature.     

Chemically modified silica gel with p-dimethylaminobenzaldehyde for selective solid-phase extraction and preconcentration of Cr(III), Cu(II), Ni(II), Pb(II) and Zn(II) by ICP-OES

Silica gel-bound amines phase modified with p-dimethylaminobenzaldehyde (p-DMABD) was prepared based on chemical immobilization technique. The product (SG-p-DMABD) was used as an adsorbent for the solid-phase extraction (SPE) Cr(III), Cu(II), Ni(II), Pb(II) and Zn(II) prior to their determination by inductively coupled plasma optical emission spectrometry (ICP-OES). The uptake behaviors of SG-p-DMABD for extracting these metal ions were studied using batch and column procedures. For the batch method, the optimum pH range for Cr(III) and Ni(II) extraction was ≥ 3, for Cu(II), Pb(II) and Zn(II) extraction it was ≥ 4. For simultaneous enrichment and determination of all the metals on the newly designed adsorbent, the pH value if 4.0 was selected. All the metal ions can be desorbed with 2.0 mL of 0.5 mol L^− 1 of HCl. The results indicate that SG-p-DMABD has rapid adsorption kinetics using the batch method. The adsorption capacity for these metal ions is in the range of 0.40-1.15 mmol g^− 1, with a high enrichment factor of 125. The presence of commonly coexisting ions does not affect the sorption capacities. The detection limits of the method were found to be 1.10, 0.69, 0.99, 1.10 and 6.50 μg L^− 1 for Cr(III), Cu(II), Ni(II), Pb(II) and Zn(II), respectively. The relative standard deviation (RSD) of the method under optimum conditions was 5.0% (n = 8) for all metal ions. The method was applied to the preconcentration of Cr(III), Cu(II), Ni(II), Pb(II) and Zn(II) from the certified reference material (GBW 08301, river sediment) and water samples with satisfactory results.     

Spectrophotometric determination of iron (II) using a highly sensitive chromogenic reaction of 4-(2-benzothiazolylazo)pyrocatechol

4-(2-Benzothiazolylazo)pyrocatechol (BTAPC), a new organic reagent, reacts with Fe(II) to form a stable, water-soluble, violet, positively charged, binary complex over a pH range of 6.3 to 6.8. Some water-miscible organic solvents and surfactants have solubilization and sensibilization effects. In 40% acetone-water medium at 25 °C, a (ionic strength) value of 0.1 and a pH value of 6.5, the ratio of Fe(II) to BTAPC in the complex is 1 2 with an apparent stability constant of 1.97 × 10⁹ and two absorption peaks at 535 and 615 nm. If the absorbance is measured at 615 nm, not only the sensitivity is high (₆₁₅ = 1.10 × 10⁵ 1 · mol^–1 · cm^–1) and the contrast () is large (90 nm), but also the selectivity is fairly good. Beer's law is obeyed for iron along the range of 0 to 16 g per 25 ml. The satisfactory results have been achieved after applying the method to the determination of traces of iron in water.